A Frequency Response Approach to Sliding Control Design for Hydraulic Drives

Sliding modes applied in control structures may generally provide for perfect control performance and robustness toward uncertain bounded parameters and disturbances, in the ideal case with infinite actuator bandwidth and switching frequency. However, in the context of physical systems, such performance cannot be realized due to finite actuator bandwidths and switching frequencies, which, in the case of direct application of sliding control terms, may lead to control chattering and high frequency oscillations in the system states. In order to compensate for this undesirable effect, the application of so-called boundary layers are commonly applied, guaranteeing sliding precision in some well-defined vicinity of the control target. Commonly the control target, or sliding manifold, is designed as some desired closed loop dynamics of the controlled plant, utilizing multiple states as feedback. However, when considering hydraulic cylinder drives, such full state feedback may not be available, and alternative approaches to conventional methods may be considered. This issue is addressed in this paper in regard to tracking control design for valve controlled hydraulic cylinder drives, and a design method taking its offset in linear analysis is proposed. The sliding manifold is designed based on a PI controller design, and the resulting controller provides for robustness outside a predefined boundary layer, and performance equivalent to the PI controller within the boundary layer. Results demonstrate improved tracking accuracy of the proposed controller compared the PI controller, and that performance of these controllers is equivalent within the boundary layer.